Like a couple of people have mentioned, comments are actually quite rare. It's the highest form of engagement. People are actually taking the hand off their mouse and taking the time to write a message. In internet terms, that's a big effort! So I don't think it's a case of having to wait x amount of time, it's more a case of having to wait x amount of views. Looking at my analytics, I get a comment on average for every 600 views. I get a higher average for the first week while my subs and most loyal viewers watch and comment, but overall, it works out at a comment every 600 views. It's a numbers game.